Precipitation Of Hydration Products Research Articles

Influence of in situ ettringite formation on the rheological properties of quartz suspensions

Cement suspensions are subject to hydration processes causing dissolution of cement clinker, precipitation of hydration products and changes in the ion content of the carrier liquid. These mechanisms take place in parallel and control the workability of cement suspensions. The precipitation of nano-granular ettringite is currently believed to be one of the key influencing factors for rheological properties of fresh cement suspensions. In order to quantify the effect of ettringite precipitation from other precipitation or dissolution processes onto the rheological behaviour, a model suspension consisting of quartz powder and in situ formed ettringite is used, enabled by implementing a new upscalable synthesis route of ettringite. The quantity of formed ettringite in the model suspension is varied and derived qualitatively from X-ray diffraction as well as quantitatively from inductively coupled plasma optical emission spectrometry and thermogravimetric analysis. The influence of the ettringite content on the rheological properties is determined and linked to the specific surface of the formed ettringite particles. This allows a quantification of the controlling mechanism of ettringite and the dependency of these mechanisms on particle properties. The correlation of the results to a reference suspension without ettringite verifies the statement of influence.

The effect of sodium citrate on NaOH-activated BFS cement: Hydration, mechanical property, and micro/nanostructure

This paper presents a study on the effect of sodium citrate solution (0.25M, 0.5M, 0.75M) on the hydration process and microstructure evolution of NaOH-activated blast furnace slag (BFS) paste. The setting behavior, flowability, mechanical strength, hydration kinetics, and micro/nanostructure characteristics were investigated through multiple tests—such as Vicat test, mini-slump test, x-ray diffraction, spectroscopy analysis, nuclear magnetic resonance, and mercury intrusion porosimetry—to reveal the hydration mechanism of NaOH-activated BFS paste in presence of sodium citrate. The results indicate that the citrate ions retarded the hydration process and contributed significantly to slag dissolution and precipitation of hydration products during the alkali-activated reaction process. The microstructure build-up from the early stage (3 days) to the mature stage (28 days) underwent a different process, resulting in a more densified and less porous matrix with higher compressive strength. However, no significant changes were observed in the type of hydration products and their nanostructural characteristics.

Time-dependent retardation effect of epoxy latexes on cement hydration: Experiments and multi-component hydration model

The kinetics and thermodynamics of the retardation effect of epoxy latexes on cement hydration were investigated using isothermal calorimetry and in-situ x-ray diffraction. The experimental results showed that the retardation effect of epoxy latexes on cement hydration was time-dependent. When the epoxy latexes were added to cement pastes, heat-generation rate was slowed, the hydration induction and acceleration periods were delayed, and the heat-flow maxima were suppressed. The addition of epoxy latexes markedly retarded silicate and aluminate hydrations by retarding both the dissolution of cement clinker minerals and the precipitation of hydration products. Higher epoxy latex content led to even slower cement hydration. Based on these results, a multi-component hydration model for composite cementitious materials, considering the time-dependent retardation effect of epoxy latexes on cement hydration, was established. The numerical results showed that the model showed great potential to predict the kinetics and thermodynamics of cement hydration with different epoxy latex contents, and provided more details about the phase evolution of the clinker minerals during cement hydration.

How do different testing procedures affect the rheological properties of cement paste?

Rheological properties of cement-based materials are shear- and time dependent due to the chemical and physical particle bonding. Proper characterization of these properties is necessary to predict the behavior of concrete over time. Often, results on rheological measurements are dependent on different preparation conditions, testing procedures and measuring instruments employed. This paper explores how changes in the testing procedure impact the rheological properties of cement pastes with different superplasticizers. Three procedures with a different shear rate profile were repeatedly imposed. The last shearing for each procedure step was identical, imposing the reference state. The results indicate that the time-evolution of the rheological properties is strongly dependent on the employed procedure. It is hypothesized that, at low imposed shear rates, dissolution and precipitation of hydration products is accelerated, creating a stronger internal structure. As a result, the imposition of a reference state does not guarantee full elimination of shear history.

Modelling the dissolution and precipitation process of the early hydration of C3S

In this study, thermodynamic and kinetic models are established to describe the early hydration of tricalcium silicate, by means of considering of dissolution and precipitation mechanisms separately. Since both the dissolution of C3S and the precipitation of hydration products are affected by the ions concentrations, these two procedures are coupled via the aqueous properties of the solution. In this model, a critical length of outer-CSH is introduced to modify the boundary nucleation and growth. The influence of hydration temperature and particle size distribution on the early hydration kinetics of C3S are evaluated with the newly developed model. A good simulation is obtained.

Investigation on early-age hydration, mechanical properties and microstructure of seawater sea sand cement mortar

Using seawater for concrete manufacturing promisingly provides significant economical and environmental benefits. In this study, ordinary Portland cement (OPC) hydration in distilled water and seawater and the corresponding evolution of solid phases was investigated by heat evolution, hydrated phase, hydration kinetics, and microstructure characterization. The results show that seawater can promote the early hydration of tricalcium silicate (C3S) during the hydration acceleration period. The hydrated phase assemblage was affected by the dissolved ions in seawater. Friedel’s salt was detected as a specific hydration phase in seawater, which was formed by chemical combination between the aluminate ferrite monosulfate (AFm) phase and chloride ions. The monocarboaluminate can be converted into a stable phase as Friedel’s salt in the seawater, due to the reaction with chloride ions. Furthermore, the ettringite becomes more stable when coexists with Friedel’s salt than that with monocarboaluminate, and thus ettringite formed in seawater remains 67% higher than that formed in distilled water at the later curing age. Moreover, additional unhydrated cement and less amorphous calcium silicate hydrate (C-S-H) were formed in seawater, which might be responsible for the slightly lower compressive strength of cement mortar prepared by seawater and sea sand. A modeled evolution of the solid phase and pore solution have been established, which agrees well with the characteristics of the dissolution of mineral phase, precipitation of hydration products and changes of pore solution. The related results can provide an insight into the applications of seawater and sea sand concrete for marine infrastructures.

Adsorption of organic core-shell corrosion inhibitors on cement particles and their influence on early age properties of fresh cement paste

The influence of copolymers on the material properties of cement-based materials is closely related to their adsorption and distribution in cement matrix. In this study, the adsorption of organic core-shell corrosion inhibitors based on poly(ethylene oxide)-b-polystyrene (PEO-b-PS) micelles on cement particles and their influence on the early age hydration and workability of cement paste were investigated. The results showed that the micelles rapidly adsorbed on cement particles surface within 40 s. At mesoscale, the micelles distribution was uniform in cement matrix; at microscale they were preferentially adsorbed on positively charged aluminates phases. Due to the efficient adsorption, the particle dissolution and precipitation of hydration products were impeded, leading to the slightly retarded early hydration of C3A; the workability of cement paste was improved, which was attributed to the better dispersion of cement particles associated with the altered surface potential of cement particles and steric hindrance effect provided by the micelles.

The role of sulfate ions in tricalcium aluminate hydration: New insights

As a major mineral of Portland cement, the hydration of tricalcium aluminate (C3A) has a significant impact on various properties of cement-based materials. However, there is little consensus on its mechanism, especially the interaction between C3A and gypsum which usually co-exist in cement systems. Recent researches have shown that sulfate ions play an important role in the kinetics of C3A hydration, which may provide more promising alternatives on controlling cement hydration besides the present solution on adjusting gypsum contents. In this study, effects of different kinds of solid sulfates (gypsum, Na2SO4, MgSO4) on the heat released during the early age of C3A hydration as well as hydration products were investigated first by isothermal calorimetry, SEM, XRD and thermal analysis. Retardation was observed in all the three systems (C3A-gypsum, C3A-Na2SO4 and C3A-MgSO4). But the duration of induction period varied with cations. Then barium nitrate (Ba(NO3)2) aiming to remove the sulfate ions was innovatively employed to highlight the role of sulfate ions. Interestingly, the renewed hydration of C3A occurred immediately after Ba(NO3)2 solution was added into the three systems with different amount of AFt remained. According to the characterization of hydration products, little monosulphoaluminate (s-AFm) was detected in the induction period of C3A-gypsum hydration and a large amount of h-AFm was found to cover the surface of C3A particles after adding Ba(NO3)2 solution. It is concluded that the coexistence of calcium and sulfate ions absorbed at C3A surface is more likely to dominate the retardation of C3A hydration in the presence of gypsum, rather than the alleged physical barrier mechanism caused by the precipitation of hydration products.

Sinterization and hydration of synthesized cement clinker doped with sulfates

This paper aims to evaluate the influence of three kinds of sulfates from the green production of cement on its sintering and hydration. The properties of clinker and hydration were monitored by thermogravimetric and differential thermal analysis (TG–DTA), X-ray diffraction, X-ray fluorescence and isothermal conduction calorimeter. Results indicate that gypsum lowers the decomposition temperature of CaCO3 and all these Sulfates will enhance the solid-phase reaction but increase melting temperature. Sulfates reduce the content of C3S, but K2SO4 and 2CaSO4·K2SO4 is conducive to the formation of β-C2S. The hydration induction period is shortened by the sulfates. K2SO4 and 2CaSO4·K2SO4 improve the early hydration of clinker, but gypsum may lightly reduce the hydration reactivity of clinker in acceleration period. 2CaSO4·K2SO and K2SO can significantly accelerate the compressive strength development of cement clinker before 3 d; by contrast, gypsum is detrimental for that. The precipitation of hydration products (CH and C–S–H) in clinker with sulfates is more than that of clinker without sulfates at 9 h. K2SO4 can accelerate the hydration of clinker without forming ettringite.

Influence of the alkaline solution and temperature on the rheology and reactivity of alkali-activated fly ash pastes

The aim of the present research is to ascertain the effect of the nature and concentration of the alkaline activator and temperature on the rheological behaviour of alkali-activated fly ash (AAFA) pastes. Furthermore, the reaction process of fly ash has been investigated from rheological measurements.Results have shown that the increase of the activator concentration and the temperature leads to an increase of yield stress and apparent viscosity, mainly at temperatures above 65 °C when dissolution of fly ash and precipitation of hydration products is raised. In general, similar activation energies were determined for alkali-activated fly ash pastes, regardless of the concentration and silica modulus of the activator, concluding that the underlying reaction mechanism is the same independently on the nature of the activating solution.

The impact of lithium nitrate on the physical and mechanical properties of Portland cement)

The effectiveness of lithium nitrate as a chemical additive which reduces the negative effects of alkali aggregate reaction was subject to research by scientists in many centres around the world. The literature data on the impact of lithium nitrate on the physical and mechanical properties of cements are rare. Without a precise definition of the impact of lithium nitrate on the cement properties, it is extremely hard to determine its real advantages in practical usage. In this paper, studies were undertaken to assess the impact of LiNO3 on the properties of pastes and mortars with Portland cement. The rate of hydration of the cement with lithium additive was examined by isothermal calorimetry, measurements of setting time and phase composition of cement pastes in the initial stages of hydration. The influence of the admixture on the compressive strength development of mortars after 2, 7 and 28 days of hardening was also researched. Results indicate that lithium nitrate accelerates the early hydration of Portland cement, affecting the precipitation of hydration products. The compressive strength of mortars with lithium admixture decrease after 28 days, although 2 an 7-day strength were greater than the control mortars.

Le Châtelier’s conjecture: Measurement of colloidal eigenstresses in chemically reactive materials

Volume changes in chemically reactive materials, such as hydrating cement, play a critical role in many engineering applications that require precise estimates of stress and pressure developments. But a means to determine bulk volume changes in the absence of other deformation mechanisms related to thermal, pressure and load variations, is still missing. Herein, we present such a measuring devise, and a hybrid experimental–theoretical technique that permits the determination of colloidal eigenstresses. Applied to cementitious materials, it is found that bulk volume changes in saturated cement pastes at constant pressure and temperature conditions result from a competition of repulsive and attractive phenomena that originate from the relative distance of the solid particles – much as Henry Louis Le Châtelier, the father of modern cement science, had conjectured in the late 19th century. Precipitation of hydration products in confined spaces entails a repulsion, whereas the concurrent reduction in interparticle distance entails activation of attractive forces in charged colloidal particles. This cross-over from repulsion to attraction can be viewed as a phase transition between a liquid state (below the solid percolation) and the limit packing of hard spheres, separated by an energy barrier that defines the temperature-dependent eigenstress magnitude.

Influence of nano-TiO2 on physical and hydration characteristics of fly ash–cement systems

This paper presents the effect of nano-TiO2 (NT) on physical and hydration characteristics of cement-based materials containing different fly ash (FA) contents ranging from 10% to 30% (by weight) as partial replacement of cement. The fluidity of mortars and the setting time of pastes were tested with different NT dosage. The compressive strength of mortars is measured at 3, 7, 28 and 90days. Results show that NT addition could accelerate initial and final setting and decrease the fluidity, while FA had the opposite effects. Introducing NT would lead to a considerable increase in compressive strength at early ages while it had an adverse effect on the later age strength. However, the positive functions of FA in late strength could offset the negative influence of NT and the optimum contents of NT and FA are 3% and 20%, respectively. Then the influence mechanisms of NT on properties of FA–cement materials are investigated by hydration heat, X-ray diffraction (XRD), differential scanning calorimetry–thermogravimetry (DSC–TG), scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). The results of analyses indicate that incorporating NT promoted the early hydration of cement and accelerated the formation and precipitation of hydration products. Furthermore, in the strength development process, NT could raise the pozzolanic reaction degree of FA and lead to the formation of a C-S-H type with longer chain and higher Al:Si ratio. Also, the interfacial transition zone (ITZ) of FA mortars were improved by NT.

Statistical relationship between mix properties and the interfacial transition zone around embedded rebar

The relationship between concrete mix properties and the properties of the interfacial transition zone (ITZ) formed around embedded rebar was investigated. Multiple samples of various mix compositions and bar orientations were prepared so as to represent common concrete technology. Water-to-cement ratios varied from 0.40 to 0.65 and powder (cement+limestone filler) contents ranged from 362kg/m3 to 564kg/m3. Over 1300 BSE images of the steel–concrete interface were taken and analyzed automatically. Statistical methods were used to identify correlations between ITZ properties and mix composition or fresh mix properties.A single large void was identified beneath all horizontal bars regardless of concrete composition. The ITZ around vertical bars was more uniform and extended around the entire rebar. No clear relationship was found between ITZ thickness and mix composition or fresh mix properties for either vertical or horizontal bar orientations. The degree of ITZ variability beneath horizontal bars clearly depends, however, on the bleeding properties of the mix. The distance from steel surface to the closest concrete solid, which influences the chemistry over the surface of the steel, is affected by precipitation of hydration products in horizontal bars, but not by mix composition.

Use of calorimetry and other methods in the studies of water reducers and set retarders interaction with hydrating cement paste

The calorimetric method was used as a main tool in the studies of the effect of superplasticizing and set retarding admixtures on cement hydration. The experiments were carried out using two types of cements (neat Portland cement CEM I 42.5R and blended Portland cement type CEM II/B-M (S-V) 42.5N), at varying w/c ratio 0.5 and 0.3 respectively. The hampering of cement hydration process at early age in the presence of these admixtures was generally observed. This is in practice equivalent to the set retardation. The dissolution, as well as the precipitation of hydration products were retarded too, as it has been proved by the DTA/TG, conductometric measurements and the evaluation of so-called chemical shrinkage. However in case of superplasticizers, after a prolonged “dormant” period the process is continued; the reactions occur at fairly good rate giving substantial heat effects – it can be concluded from the total heat evolved values after standard 41h. At longer time a significant amounts of products are formed as well, but it seems that the crystallization of calcium oxide is disturbed; simultaneously the amount and composition of calcium silicate hydrates alters towards the “more amorphous” product. This could be observed under SEM. The set retarders give strong delaying effect, particularly at low amount of water and in the pastes produced from Portland cement. This can be due to the formation of insoluble layer of calcium and sugar or calcium and phosphorus containing compounds surrounding the hydrating grains; the formation of this “barrier” can be distinguished as a small peak on the calorimetric curve and it is poorly removable on further hydration. The shrinkage and conductivity measurements seem to prove not only the change of kinetics of heat evolution/hydration but also the modification of the process and resulting products.

Microstructural observations in new matrix glass fibre reinforced cement

Glass fibre reinforced cement (GRC) durability is generally thought to be governed by weakening of the reinforcement by the alkaline matrix and/or precipitation of hydration products, particularly calcium hydroxide (CH), within and around the strands. Previous microstructural investigations of GRC have exclusively used scanning electron microscopy (SEM). This paper is concerned with the application of thin-section petrography (TSP) to GRC. TSP allows identification of CH and other phases associated with the strands and unlike SEM, sample preparation causes minimal interfacial disturbance. Three matrices were studied; OPC, OPC plus metakaolin and OPC plus calcium sulphoaluminate cement. It was found that the degree of composite degradation was unrelated to the amount of CH or other hydration products precipitated within the strands or at the interface. No significant loss of fibre section was observed in degraded composites. It is postulated that gradual enlargement of pre-existing fibre flaws causes strength loss in GRC.